A nephron-based model of the kidneys for macro-to-micro α-particle dosimetry

ISIT-QA: In Silico Imaging Trial to Evaluate a Low-Count Quantitative SPECT Method Across Multiple Scanner-Collimator Configurations for 223Ra-Based Radiopharmaceutical Therapies

Personalized dose-based treatment planning requires accurate and reproducible noninvasive measurements to ensure safety and effectiveness. Dose estimation using SPECT is possible but challenging for alpha (α)-particle-emitting radiopharmaceutical therapy (α-RPT) because of complex γ-emission spectra, extremely low counts, and various image-degrading artifacts across a plethora of scanner-collimator configurations. Through the incorporation of physics-based considerations and skipping of the potentially lossy voxel-based reconstruction step, a recently developed projection-domain low-count quantitative SPECT (LC-QSPECT) method has the potential to provide reproducible, accurate, and precise activity concentration and dose measures across multiple scanners, as is typically the case in multicenter settings. To assess this potential, we conducted an in silico imaging trial to evaluate the LC-QSPECT method for a 223Ra-based α-RPT, with the trial recapitulating patient and imaging system variabilities. Methods: A virtual imaging trial titled In Silico Imaging Trial for Quantitation Accuracy (ISIT-QA) was designed with the objectives of evaluating the performance of the LC-QSPECT method across multiple scanner-collimator configurations and comparing performance with a conventional reconstruction-based quantification method. In this trial, we simulated 280 realistic virtual patients with bone-metastatic castration-resistant prostate cancer treated with 223Ra-based α-RPT. The trial was conducted with 9 simulated SPECT scanner-collimator configurations. The primary objective of this trial was to evaluate the reproducibility of dose estimates across multiple scanner-collimator configurations using LC-QSPECT by calculating the intraclass correlation coefficient. Additionally, we compared the reproducibility and evaluated the accuracy of both considered quantification methods across multiple scanner-collimator configurations. Finally, the repeatability of the methods was evaluated in a test-retest study. Results: In this trial, data from 268 223RaCl2 treated virtual prostate cancer patients, with a total of 2,903 lesions, were used to evaluate LC-QSPECT. LC-QSPECT provided dose estimates with good reproducibility across the 9 scanner-collimator configurations (intraclass correlation coefficient > 0.75) and high accuracy (ensemble average values of recovery coefficients ranged from 1.00 to 1.02). Compared with conventional reconstruction-based quantification, LC-QSPECT yielded significantly improved reproducibility across scanner-collimator configurations, accuracy, and test-retest repeatability ([Formula: see text] Conclusion: LC-QSPECT provides reproducible, accurate, and repeatable dose estimations in 223Ra-based α-RPT as evaluated in ISIT-QA. These findings provide a strong impetus for multicenter clinical evaluations of LC-QSPECT in dose quantification for α-RPTs.

EANM guidance document: dosimetry for first-in-human studies and early phase clinical trials

The numbers of diagnostic and therapeutic nuclear medicine agents under investigation are rapidly increasing. Both novel emitters and novel carrier molecules require careful selection of measurement procedures. This document provides guidance relevant to dosimetry for first-in human and early phase clinical trials of such novel agents. The guideline includes a short introduction to different emitters and carrier molecules, followed by recommendations on the methods for activity measurement, pharmacokinetic analyses, as well as absorbed dose calculations and uncertainty analyses. The optimal use of preclinical information and studies involving diagnostic analogues is discussed. Good practice reporting is emphasised, and relevant dosimetry parameters and method descriptions to be included are listed. Three examples of first-in-human dosimetry studies, both for diagnostic tracers and radionuclide therapies, are given.

Pre-clinical evaluation of biomarkers for the early detection of nephrotoxicity following alpha-particle radioligand therapy

PURPOSE

Cancer treatment with alpha-emitter-based radioligand therapies (α-RLTs) demonstrates promising tumor responses. Radiolabeled peptides are filtered through glomeruli, followed by potential reabsorption of a fraction by proximal tubules, which may cause acute kidney injury (AKI) and chronic kidney disease (CKD). Because tubular cells are considered the primary site of radiopeptides' renal reabsorption and potential injury, the current use of kidney biomarkers of glomerular functional loss limits the evaluation of possible nephrotoxicity and its early detection. This study aimed to investigate whether urinary secretion of tubular injury biomarkers could be used as an additional non-invasive sensitive diagnostic tool to identify unrecognizable tubular damage and risk of long-term α-RLT nephrotoxicity.

METHODS

A bifunctional cyclic peptide, melanocortin 1 ligand (MC1L), labeled with [203Pb]Pb-MC1L, was used for [212Pb]Pb-MC1L biodistribution and absorbed dose measurements in CD-1 Elite mice. Mice were treated with [212Pb]Pb-MC1L in a dose-escalation study up to levels of radioactivity intended to induce kidney injury. The approach enabled prospective kidney functional and injury biomarker evaluation and late kidney histological analysis to validate these biomarkers.

RESULTS

Biodistribution analysis identified [212Pb]Pb-MC1L reabsorption in kidneys with a dose deposition of 2.8, 8.9, and 20 Gy for 0.9, 3.0, and 6.7 MBq injected [212Pb]Pb-MC1L doses, respectively. As expected, mice receiving 6.7 MBq had significant weight loss and CKD evidence based on serum creatinine, cystatin C, and kidney histological alterations 28 weeks after treatment. A dose-dependent urinary neutrophil gelatinase-associated lipocalin (NGAL, tubular injury biomarker) urinary excretion the day after [212Pb]Pb-MC1L treatment highly correlated with the severity of late tubulointerstitial injury and histological findings.

CONCLUSION

Urine NGAL secretion could be a potential early diagnostic tool to identify unrecognized tubular damage and predict long-term α-RLT-related nephrotoxicity.

Pre-clinical Evaluation of Biomarkers for Early Detection of Nephrotoxicity Following Alpha-particle Radioligand Therapy

Purpose

Cancer treatment with alpha-emitter-based radioligand therapies (α-RLTs) demonstrates promising tumor responses. Radiolabeled peptides are filtered through glomeruli, followed by potential reabsorption of a fraction by proximal tubules, which may cause acute kidney injury (AKI) and chronic kidney disease (CKD). Because tubular cells are considered the primary site of radiopeptides' renal reabsorption and potential injury, the current use of kidney biomarkers of glomerular functional loss limits the evaluation of possible nephrotoxicity and its early detection. This study aimed to investigate whether urinary secretion of tubular injury biomarkers could be used as additional non-invasive sensitive diagnostic tool to identify unrecognizable tubular damage and risk of long-term α-RLTs nephrotoxicity.

Methods

A bifunctional cyclic peptide, melanocortin ligand-1(MC1L), labeled with [ 203 Pb]Pb-MC1L, was used for [ 212 Pb]Pb-MC1L biodistribution and absorbed dose measurements in CD-1 Elite mice. Mice were treated with [ 212 Pb]Pb-MC1L in a dose escalation study up to levels of radioactivity intended to induce kidney injury. The approach enabled prospective kidney functional and injury biomarker evaluation and late kidney histological analysis to validate these biomarkers.

Results

Biodistribution analysis identified [ 212 Pb]Pb-MC1L reabsorption in kidneys with a dose deposition of 2.8, 8.9, and 20 Gy for 0.9, 3.0, and 6.7 MBq injected [ 212 Pb]Pb-MC1L doses, respectively. As expected, mice receiving 6.7 MBq had significant weight loss and CKD evidence based on serum creatinine, cystatin C, and kidney histological alterations 28 weeks after treatment. A dose-dependent urinary Neutrophil gelatinase-associated lipocalin (NGAL, tubular injury biomarker) urinary excretion the day after [ 212 Pb]Pb-MC1L treatment highly correlated with the severity of late tubulointerstitial injury and histological findings.

Conclusion

urine NGAL secretion could be a potential early diagnostic tool to identify unrecognized tubular damage and predict long-term α-RLT-related nephrotoxicity.

[212Pb]Pb-eSOMA-01: A Promising Radioligand for Targeted Alpha Therapy of Neuroendocrine Tumors

Peptide receptor radionuclide therapy (PRRT) has been applied to the treatment of neuroendocrine tumors (NETs) for over two decades. However, improvement is still needed, and targeted alpha therapy (TAT) with alpha emitters such as lead-212 (²¹²Pb) represents a promising avenue. A series of ligands based on octreotate was developed. Lead-203 was used as an imaging surrogate for the selection of the best candidate for the studies with lead-212. ^203/212Pb radiolabeling and in vitro assays were carried out, followed by SPECT/CT imaging and ex vivo biodistribution in NCI-H69 tumor-bearing mice. High radiochemical yields (≥99%) and purity (≥96%) were obtained for all ligands. [²⁰³Pb]Pb-eSOMA-01 and [²⁰³Pb]Pb-eSOMA-02 showed high stability in PBS and mouse serum up to 24 h, whereas [²⁰³Pb]Pb-eSOMA-03 was unstable in those conditions. All compounds exhibited a nanomolar affinity (2.5-3.1 nM) for SSTR2. SPECT/CT images revealed high tumor uptake at 1, 4, and 24 h post-injection of [²⁰³Pb]Pb-eSOMA-01/02. Ex vivo biodistribution studies confirmed that the highest uptake in tumors was observed with [²¹²Pb]Pb-eSOMA-01. [²¹²Pb]Pb-eESOMA-01 displayed the highest absorbed dose in the tumor (35.49 Gy/MBq) and the lowest absorbed dose in the kidneys (121.73 Gy/MBq) among the three tested radioligands. [²¹²Pb]Pb-eSOMA-01 is a promising candidate for targeted alpha therapy of NETs. Further investigations are required to confirm its potential.

First preclinical evaluation of [225Ac]Ac-DOTA-JR11 and comparison with [177Lu]Lu-DOTA-JR11, alpha versus beta radionuclide therapy of NETs

BACKGROUND

The [¹⁷⁷Lu]Lu-DOTA-TATE mediated peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors (NETs) is sometimes leading to treatment resistance and disease recurrence. An interesting alternative could be the somatostatin antagonist, [¹⁷⁷Lu]Lu-DOTA-JR11, that demonstrated better biodistribution profile and higher tumor uptake than [¹⁷⁷Lu]Lu-DOTA-TATE. Furthermore, treatment with alpha emitters showed improvement of the therapeutic index of PRRT due to the high LET offered by the alpha particles compared to beta emitters. Therefore, [²²⁵Ac]Ac-DOTA-JR11 can be a potential candidate to improve the treatment of NETs (Graphical abstract). DOTA-JR11 was radiolabeled with [²²⁵Ac]Ac(NO₃)₃ and [¹⁷⁷Lu]LuCl₃. Stability studies were performed in phosphate buffered saline (PBS) and mouse serum. In vitro competitive binding assay has been carried out in U2OS-SSTR2 + cells for ^natLa-DOTA-JR11, ^natLu-DOTA-JR11 and DOTA-JR11. Ex vivo biodistribution studies were performed in mice inoculated with H69 cells at 4, 24, 48 and 72 h after injection of [²²⁵Ac]Ac-DOTA-JR11. A blocking group was included to verify uptake specificity. Dosimetry of selected organs was determined for [²²⁵Ac]Ac-DOTA-JR11 and [¹⁷⁷Lu]Lu-DOTA-JR11.

RESULTS

[²²⁵Ac]Ac-DOTA-JR11 has been successfully prepared and obtained in high radiochemical yield (RCY; 95%) and radiochemical purity (RCP; 94%). [²²⁵Ac]Ac-DOTA-JR11 showed reasonably good stability in PBS (77% intact radiopeptide at 24 h after incubation) and in mouse serum (~ 81% intact radiopeptide 24 h after incubation). [¹⁷⁷Lu]Lu-DOTA-JR11 demonstrated excellent stability in both media (> 93%) up to 24 h post incubation. Competitive binding assay revealed that complexation of DOTA-JR11 with ^natLa and ^natLu did not affect its binding affinity to SSTR2. Similar biodistribution profiles were observed for both radiopeptides, however, higher uptake was noticed in the kidneys, liver and bone for [²²⁵Ac]Ac-DOTA-JR11 than [¹⁷⁷Lu]Lu-DOTA-JR11.

CONCLUSION

[²²⁵Ac]Ac-DOTA-JR11 showed a higher absorbed dose in the kidneys compared to [¹⁷⁷Lu]Lu-DOTA-JR11, which may limit further studies with this radiopeptide. However, several strategies can be explored to reduce nephrotoxicity and offer opportunities for future clinical investigations with [²²⁵Ac]Ac-DOTA-JR11.

Heterogeneity of absorbed dose distribution in kidney tissues and dose–response modelling of nephrotoxicity in radiopharmaceutical therapy with beta-particle emitters: A review

Absorbed dose heterogeneity in kidney tissues is an important issue in radiopharmaceutical therapy. The effect of absorbed dose heterogeneity in nephrotoxicity is, however, not fully understood yet, which hampers the implementation of treatment optimization by obscuring the interpretation of clinical response data and the selection of optimal treatment options. Although some dosimetry methods have been developed for kidney dosimetry to the level of microscopic renal substructures, the clinical assessment of the microscopic distribution of radiopharmaceuticals in kidney tissues currently remains a challenge. This restricts the anatomical resolution of clinical dosimetry, which hinders a thorough clinical investigation of the impact of absorbed dose heterogeneity. The potential of absorbed dose-response modelling to support individual treatment optimization in radiopharmaceutical therapy is recognized and gaining attraction. However, biophysical modelling is currently underexplored for the kidney, where particular modelling challenges arise from the convolution of a complex functional organization of renal tissues with the function-mediated dose distribution of radiopharmaceuticals. This article reviews and discusses the heterogeneity of absorbed dose distribution in kidney tissues and the absorbed dose-response modelling of nephrotoxicity in radiopharmaceutical therapy. The review focuses mainly on the peptide receptor radionuclide therapy with beta-particle emitting somatostatin analogues, for which the scientific literature reflects over two decades of clinical experience. Additionally, detailed research perspectives are proposed to address various identified challenges to progress in this field.

Radionuclides for Targeted Therapy: Physical Properties

A search in PubMed revealed that 72 radionuclides have been considered for molecular or functional targeted radionuclide therapy. As radionuclide therapies increase in number and variations, it is important to understand the role of the radionuclide and the various characteristics that can render it either useful or useless. This review focuses on the physical characteristics of radionuclides that are relevant for radionuclide therapy, such as linear energy transfer, relative biological effectiveness, range, half-life, imaging properties, and radiation protection considerations. All these properties vary considerably between radionuclides and can be optimised for specific targets. Properties that are advantageous for some applications can sometimes be drawbacks for others; for instance, radionuclides that enable easy imaging can introduce more radiation protection concerns than others. Similarly, a long radiation range is beneficial in targets with heterogeneous uptake, but it also increases the radiation dose to tissues surrounding the target, and, hence, a shorter range is likely more beneficial with homogeneous uptake. While one cannot select a collection of characteristics as each radionuclide comes with an unchangeable set, all the 72 radionuclides investigated for therapy—and many more that have not yet been investigated—provide numerous sets to choose between.

EANM dosimetry committee recommendations for dosimetry of 177Lu-labelled somatostatin-receptor- and PSMA-targeting ligands

The purpose of the EANM Dosimetry Committee is to provide recommendations and guidance to scientists and clinicians on patient-specific dosimetry. Radiopharmaceuticals labelled with lutetium-177 (177Lu) are increasingly used for therapeutic applications, in particular for the treatment of metastatic neuroendocrine tumours using ligands for somatostatin receptors and prostate adenocarcinoma with small-molecule PSMA-targeting ligands. This paper provides an overview of reported dosimetry data for these therapies and summarises current knowledge about radiation-induced side effects on normal tissues and dose-effect relationships for tumours. Dosimetry methods and data are summarised for kidneys, bone marrow, salivary glands, lacrimal glands, pituitary glands, tumours, and the skin in case of radiopharmaceutical extravasation. Where applicable, taking into account the present status of the field and recent evidence in the literature, guidance is provided. The purpose of these recommendations is to encourage the practice of patient-specific dosimetry in therapy with 177Lu-labelled compounds. The proposed methods should be within the scope of centres offering therapy with 177Lu-labelled ligands for somatostatin receptors or small-molecule PSMA.

Imaging and dosimetry for alpha-particle emitter radiopharmaceutical therapy: improving radiopharmaceutical therapy by looking into the black box

Radiopharmaceutical therapy using α-particle emitting radionuclides (αRPT) is a novel treatment modality that delivers highly potent alpha-particles to cancer cells or their environment. We review the advantages and challenges of imaging and dosimetry in implementing αRPT for cancer patients.

Role of Artificial Intelligence in Theranostics:: Toward Routine Personalized Radiopharmaceutical Therapies

We highlight emerging uses of artificial intelligence (AI) in the field of theranostics, focusing on its significant potential to enable routine and reliable personalization of radiopharmaceutical therapies (RPTs). Personalized RPTs require patient-specific dosimetry calculations accompanying therapy. Additionally we discuss the potential to exploit biological information from diagnostic and therapeutic molecular images to derive biomarkers for absorbed dose and outcome prediction; toward personalization of therapies. We try to motivate the nuclear medicine community to expand and align efforts into making routine and reliable personalization of RPTs a reality.

Individual dosimetry system for targeted alpha therapy based on PHITS coupled with microdosimetric kinetic model

BACKGROUND

An individual dosimetry system is essential for the evaluation of precise doses in nuclear medicine. The purpose of this study was to develop a system for calculating not only absorbed doses but also EQDX(α/β) from the PET-CT images of patients for targeted alpha therapy (TAT), considering the dose dependence of the relative biological effectiveness, the dose-rate effect, and the dose heterogeneity.

METHODS

A general-purpose Monte Carlo particle transport code PHITS was employed as the dose calculation engine in the system, while the microdosimetric kinetic model was used for converting the absorbed dose to EQDX(α/β). PHITS input files for describing the geometry and source distribution of a patient are automatically created from PET-CT images, using newly developed modules of the radiotherapy package based on PHITS (RT-PHITS). We examined the performance of the system by calculating several organ doses using the PET-CT images of four healthy volunteers after injecting 18F-NKO-035.

RESULTS

The deposition energy map obtained from our system seems to be a blurred image of the corresponding PET data because annihilation γ-rays deposit their energies rather far from the source location. The calculated organ doses agree with the corresponding data obtained from OLINDA 2.0 within 20%, indicating the reliability of our developed system. Test calculations by replacing the labeled radionuclide from 18F to 211At suggest that large dose heterogeneity in a target volume is expected in TAT, resulting in a significant decrease of EQDX(α/β) for higher-activity injection.

CONCLUSIONS

As an extension of RT-PHITS, an individual dosimetry system for nuclear medicine was developed based on PHITS coupled with the microdosimetric kinetic model. It enables us to predict the therapeutic and side effects of TAT based on the clinical data largely available from conventional external radiotherapy.

Dosimetry for Optimized, Personalized Radiopharmaceutical Therapy

Radiopharmaceutical therapy (RPT) has grown rapidly over the last decade for treatment of numerous cancer types. Dosimetric guidance, as with other radiotherapy modalities, has benefitted patients by reducing the incidence of side effects and improving overall survival in populations treated under this paradigm. Development of tools and techniques for dosimetry-guided therapy is ongoing, with numerous the Food and Drug Administration-cleared products reaching the U.S. market in 2019. Safe use of commercial dosimetry platforms requires a deep understanding of the underlying physical principles and thoroughly vetted input data. Likewise, interpretation of dosimetry results relies on an understanding of radiobiological principles, and the principles of uncertainty propagation. In this article, we review strategies commonly employed for dosimetry-guided RPT - including quantitative imaging, dose calculation methods, and modeling of dose across time-points. Additionally, we review recent literature evidence (2013-2020) demonstrating the efficacy of personalized RPT.

Radiopharmaceutical Chemistry and Drug Development-What's Changed?

Radiation oncologists and nuclear medicine physicians have seen a resurgence in the clinical use of radiopharmaceuticals for the curative or palliative treatment of cancer. To enable the discovery and the development of new targeted radiopharmaceutical treatments, the United States National Cancer Institute has adapted its clinical trial enterprise to accommodate the requirements of a development program with investigational agents that have a radioactive isotope as part of the studied drug product. One change in perspective has been the consideration of investigational radiopharmaceuticals as drugs, with maximum tolerable doses determined by normal organ toxicity frequency like in drug clinical trials. Other changes include new clinical trial enterprise elements for biospecimen handling, adverse event reporting, regulatory conduct, writing services, drug master files, and reporting of patient outcomes. Arising from this enterprise, the study and clinical use of alpha-particle and beta-particle emitters have emerged as an important approach to cancer treatment. Resources allocated to this enterprise have brought forward biomarkers of molecular pathophysiology now used to select treatment or to evaluate clinical performance of radiopharmaceuticals. The clinical use of diagnostic and therapeutic radionuclide pairs is anticipated to accelerate radiopharmaceutical clinical development.

A microdosimetry model of kidney by GATE Monte Carlo simulation using a nonuniform activity distribution in digital phantom of nephron

OBJECTIVES

As the main pathway for the clearance of radiopharmaceutical from the body, kidney is a dose-limiting organ in medical application of radionuclides. Because of its unique physiology, radioactivity is seen to concentrate on kidney nonuniformly. This nonuniformity can be considered in nephron microstructures. A microdosimetry model of kidney is necessary to include the nonuniform distribution in internal radiation dosimetry.

METHOD

Implementing the microdosimetry model requires, first, a geometry phantom of nephrons. Stylized phantoms cannot distribute activities inside nephron compartments nonuniformly. A phantom of nephron was generated by a preliminary three-dimensional graphic model and was converted to a proper format of digital phantom. The phantom was fed to GATE Monte Carlo toolkits. Simulations were performed and S-values for five radionuclides (Tc-99m, In-111, Lu-177, Ac-225 and Bi-212) were calculated and compared with corresponding results published in the literature derived with a stylized phantom of nephron. Activity was distributed nonuniformly according to the kinetics of two mainly used diagnostic tracers (diethylenetriaminepetaacetate and ethylenedicysteine) and absorbed dose of nephron cells were calculated.

RESULTS

A good correlation was shown between the generated phantom microdosimetry model and stylized model and revealed the phantom can be used for future microdosimetry studies of kidney to evaluate radiobiological effects of internal radiation from various diagnostic and therapeutic radiopharmaceuticals. Absorbed dose of cells for nonuniform distribution showed that some cells in a nephron compartment receive higher dose than (more than two-fold) that of compartment average dose.

CONCLUSION

Average dose of nephron is not a reliable parameter for nephrotoxicity evaluation.

Current Status of Radiopharmaceutical Therapy

In radiopharmaceutical therapy (RPT), a radionuclide is systemically or locally delivered with the goal of targeting and delivering radiation to cancer cells while minimizing radiation exposure to untargeted cells. Examples of current RPTs include thyroid ablation with the administration of ¹³¹I, treatment of liver cancer with ⁹⁰Y microspheres, the treatment of bony metastases with ²²³Ra, and the treatment of neuroendocrine tumors with ¹⁷⁷Lu-DOTATATE. New RPTs are being developed where radionuclides are incorporated into systemic targeted therapies. To assure that RPT is appropriately implemented, advances in targeting need to be matched with advances in quantitative imaging and dosimetry methods. Currently, radiopharmaceutical therapy is administered by intravenous or locoregional injection, and the treatment planning has typically been implemented like chemotherapy, where the activity administered is either fixed or based on a patient's body weight or body surface area. RPT pharmacokinetics are measurable by quantitative imaging and are known to vary across patients, both in tumors and normal tissues. Therefore, fixed or weight-based activity prescriptions are not currently optimized to deliver a cytotoxic dose to targets while remaining within the tolerance dose of organs at risk. Methods that provide dose estimates to individual patients rather than to reference geometries are needed to assess and adjust the injected RPT dose. Accurate doses to targets and organs at risk will benefit the individual patients and decrease uncertainties in clinical trials. Imaging can be used to measure activity distribution in vivo, and this information can be used to determine patient-specific treatment plans where the dose to the targets and organs at risk can be calculated. The development and adoption of imaging-based dosimetry methods is particularly beneficial in early clinical trials. In this work we discuss dosimetric accuracy needs in modern radiation oncology, uncertainties in the dosimetry in RPT, and best approaches for imaging and dosimetry of internal radionuclide therapy.

Targeted alpha therapy: a critical review of translational dosimetry research with emphasis on actinium-225

This review provides a general overview of the current achievements and challenges in translational dosimetry for targeted alpha therapy (TAT). The concept of targeted radionuclide therapy (TRNT) is described with an overview of its clinical applicability and the added value of TAT is discussed. For TAT, we focused on actinium-225 (225Ac) as an example for alpha particle emitting radionuclides and their features, such as limited range within tissue and high linear energy transfer, which make alpha particle emissions more effective in targeted killing of tumour cells compared to beta radiation. Starting with the state-of-the-art dosimetry for TRNT and TAT, we then describe the challenges that still need to be met in order to move to a personalized dosimetry approach for TAT. Specifically for 225Ac, we discuss the recoiled daughter effect which may provoke significant damage to healthy tissue or organs and should be considered. Next, a broad overview is given of the pre-clinical research on 225Ac-TAT with an extensive description of tools which are only available in a pre-clinical setting and their added value. In addition, we review the preclinical biodistribution and dosimetry studies that have been performed on TAT-agents and more specifically of 225Ac and its multiple progeny, and describe their potential role to better characterize the pharmacokinetic (PK) profile of TAT-agents and to optimize the use of theranostic approaches for dosimetry. Finally, we discuss the support pre-clinical studies may provide in understanding dose-effect relationships, linking radiation dose quantities to biological endpoints and even moving away from macro- to microdosimetry. As such, the translation of pre-clinical findings may provide valuable information and new approaches for improved clinical dosimetry, thus paving the way to personalized TAT.

Dosimetry, Radiobiology and Synthetic Lethality: Radiopharmaceutical Therapy (RPT) With Alpha-Particle-Emitters

As a treatment modality that is fundamentally different from other therapies against cancer, radiopharmaceutical therapy with alpha-particle emitters has drawn the attention of the therapy community and also the biopharmaceutical industry. Alpha-particles cause a preponderance of complex DNA double-strand breaks (DSBs). This provides an opportunity to either enhance cell kill by using DNA DSB repair inhibitors or identify patients who are likely to be high responders to alpha-emitter RPT. The short-range and high potency of alpha-particles requires special dosimetry considerations. These are reviewed in light of recent updates to the phantoms and associated dosimetric quantities used for dosimetry calculations. A formalism for obtaining the necessary microscale pharmacokinetic information from patient nuclear medicine imaging is presented. Alpha-emitter based radiopharmaceutical therapy is an exciting cancer therapy modality that is being revisited. Further development of imaging and dosimetric methods specific to alpha-particle emitters, coupled with standardization of the methods and rigorous evidence that dosimetry applied to alphaRPT improves patient care are needed moving forward.

Auger radiopharmaceutical therapy targeting prostate-specific membrane antigen in a micrometastatic model of prostate cancer

Auger radiopharmaceutical therapy is a promising strategy for micrometastatic disease given high linear energy transfer and short range in tissues, potentially limiting normal tissue toxicities. We previously demonstrated anti-tumor efficacy of a small-molecule Auger electron emitter targeting the prostate-specific membrane antigen (PSMA), 2-[3-[1-carboxy-5-(4-[¹²⁵I]iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid), or ¹²⁵I-DCIBzL, in a mouse xenograft model. Here, we investigated the therapeutic efficacy, long-term toxicity, and biodistribution of ¹²⁵I-DCIBzL in a micrometastatic model of prostate cancer (PC).

Methods: To test the therapeutic efficacy of ¹²⁵I-DCIBzL in micrometastatic PC, we used a murine model of human metastatic PC in which PSMA+ PC3-ML cells expressing firefly luciferase were injected intravenously in NSG mice to form micrometastatic deposits. One week later, 0, 0.37, 1.85, 3.7, 18.5, 37, or 111 MBq of ¹²⁵I-DCIBzL was administered (intravenously). Metastatic tumor burden was assessed using bioluminescence imaging (BLI). Long-term toxicity was evaluated via serial weights and urinalysis of non-tumor-bearing mice over a 12-month period, as well as final necropsy.

Results: In the micrometastatic PC model, activities of 18.5 MBq ¹²⁵I-DCIBzL and above significantly delayed development of detectable metastatic disease by BLI and prolonged survival in mice. Gross metastases were detectable in control mice and those treated with 0.37-3.7 MBq ¹²⁵I-DCIBzL at a median of 2 weeks post-treatment, versus 4 weeks for those treated with 18.5-111 MBq ¹²⁵I-DCIBzL (P<0.0001 by log-rank test). Similarly, treatment with ≥18.5 MBq ¹²⁵I-DCIBzL yielded a median survival of 11 weeks, compared with 6 weeks for control mice (P<0.0001). At 12 months, there was no appreciable toxicity via weight, urinalysis, or necropsy evaluation in mice treated with any activity of ¹²⁵I-DCIBzL, which represents markedly less toxicity than the analogous PSMA-targeted α-particle emitter. Macro-to-microscale dosimetry modeling demonstrated lower absorbed dose in renal cell nuclei versus tumor cell nuclei due to lower levels of drug uptake and cellular internalization in combination with the short range of Auger emissions.

Conclusion: PSMA-targeted radiopharmaceutical therapy with the Auger emitter ¹²⁵I-DCIBzL significantly delayed development of detectable metastatic disease and improved survival in a micrometastatic model of PC, with no long-term toxicities noted at 12 months, suggesting a favorable therapeutic ratio for treatment of micrometastatic PC.

203/212Pb Theranostic Radiopharmaceuticals for Image-guided Radionuclide Therapy for Cancer

Receptor-targeted image-guided Radionuclide Therapy (TRT) is increasingly recognized as a promising approach to cancer treatment. In particular, the potential for clinical translation of receptor-targeted alpha-particle therapy is receiving considerable attention as an approach that can improve outcomes for cancer patients. Higher Linear-energy Transfer (LET) of alpha-particles (compared to beta particles) for this purpose results in an increased incidence of double-strand DNA breaks and improved-localized cancer-cell damage. Recent clinical studies provide compelling evidence that alpha-TRT has the potential to deliver a significantly more potent anti-cancer effect compared with beta-TRT. Generator-produced 212Pb (which decays to alpha emitters 212Bi and 212Po) is a particularly promising radionuclide for receptor-targeted alpha-particle therapy. A second attractive feature that distinguishes 212Pb alpha-TRT from other available radionuclides is the possibility to employ elementallymatched isotope 203Pb as an imaging surrogate in place of the therapeutic radionuclide. As direct non-invasive measurement of alpha-particle emissions cannot be conducted using current medical scanner technology, the imaging surrogate allows for a pharmacologically-inactive determination of the pharmacokinetics and biodistribution of TRT candidate ligands in advance of treatment. Thus, elementally-matched 203Pb labeled radiopharmaceuticals can be used to identify patients who may benefit from 212Pb alpha-TRT and apply appropriate dosimetry and treatment planning in advance of the therapy. In this review, we provide a brief history on the use of these isotopes for cancer therapy; describe the decay and chemical characteristics of 203/212Pb for their use in cancer theranostics and methodologies applied for production and purification of these isotopes for radiopharmaceutical production. In addition, a medical physics and dosimetry perspective is provided that highlights the potential of 212Pb for alpha-TRT and the expected safety for 203Pb surrogate imaging. Recent and current preclinical and clinical studies are presented. The sum of the findings herein and observations presented provide evidence that the 203Pb/212Pb theranostic pair has a promising future for use in radiopharmaceutical theranostic therapies for cancer.

Preclinical Evaluation of 203/212Pb-Labeled Low-Molecular-Weight Compounds for Targeted Radiopharmaceutical Therapy of Prostate Cancer

Targeted radiopharmaceutical therapy (TRT) using α-particle radiation is a promising approach for treating both large and micrometastatic lesions. We developed prostate-specific membrane antigen (PSMA)-targeted low-molecular-weight agents for ²¹²Pb-based TRT of patients with prostate cancer (PC) by evaluating the matching γ-emitting surrogate, ²⁰³Pb. Methods: Five rationally designed low-molecular-weight ligands (L1-L5) were synthesized using the lysine-urea-glutamate scaffold, and PSMA inhibition constants were determined. Tissue biodistribution and SPECT/CT imaging of ²⁰³Pb-L1-²⁰³Pb-L5 were performed on mice bearing PSMA(+) PC3 PIP and PSMA(-) PC3 flu flank xenografts. The absorbed radiation dose of the corresponding ²¹²Pb-labeled analogs was determined using the biodistribution data. Antitumor efficacy of ²¹²Pb-L2 was evaluated in PSMA(+) PC3 PIP and PSMA(-) PC3 flu tumor models and in the PSMA(+) luciferase-expressing micrometastatic model. ²¹²Pb-L2 was also evaluated for dose-escalated, long-term toxicity. Results: All new ligands were obtained in high yield and purity. PSMA inhibitory activities ranged from 0.10 to 17 nM. ²⁰³Pb-L1-²⁰³Pb-L5 were synthesized in high radiochemical yield and specific activity. Whole-body clearance of ²⁰³Pb-L1-²⁰³Pb-L5 was fast. The absorbed dose coefficients (mGy/kBq) of the tumor and kidneys were highest for ²⁰³Pb-L5 (31.0, 15.2) and lowest for ²⁰³Pb-L2 (8.0, 4.2). The tumor-to-kidney absorbed dose ratio was higher for ²⁰³Pb-L3 (3.2) and ²⁰³Pb-L4 (3.6) than for the other agents, but with lower tumor-to-blood ratios. PSMA(+) tumor lesions were visualized through SPECT/CT as early as 0.5 h after injection. A proof-of-concept therapy study with a single administration of ²¹²Pb-L2 demonstrated dose-dependent inhibition of tumor growth in the PSMA(+) flank tumor model. ²¹²Pb-L2 also demonstrated an increased survival benefit in the micrometastatic model compared with ¹⁷⁷Lu-PSMA-617. Long-term toxicity studies in healthy, immunocompetent CD-1 mice revealed kidney as the dose-limiting organ. Conclusion: ²⁰³Pb-L1-²⁰³Pb-L5 demonstrated favorable pharmacokinetics for ²¹²Pb-based TRT. The antitumor efficacy of ²¹²Pb-L2 supports the corresponding ²⁰³Pb/²¹²Pb theranostic pair for PSMA-based α-particle TRT in advanced PC.

Radiopharmaceutical Therapy

Radiopharmaceutical therapy involves the use of radionuclides that are either conjugated to tumor-targeting agents (e.g., nanoscale constructs, antibodies, peptides, and small molecules) or that concentrate in tumors through natural physiological mechanisms that occur predominantly in neoplastic cells. In the latter category, radioiodine therapy of thyroid cancer is the prototypical and most widely implemented radiopharmaceutical therapy. In the category of radionuclide-ligand conjugates, antibody and peptide conjugates have been studied extensively. The efficacy of radiopharmaceutical therapy relies on the ability to deliver cytotoxic radiation to tumor cells without causing prohibitive normal tissue toxicity. After some 30 y of preclinical and clinical research, a number of recent developments suggest that radiopharmaceutical therapy is poised to emerge as an important and widely recognized therapeutic modality. These developments include the substantial investment in antibodies by the pharmaceutical industry and the compelling rationale to build upon this already existing and widely tested platform. In addition, the growing recognition that the signaling pathways responsible for tumor cell survival and proliferation are less easily and durably inhibited than originally envisioned has also provided a rationale for identifying agents that are cytotoxic rather than inhibitory. A number of radiopharmaceutical agents are currently undergoing clinical trial investigation; these include beta-particle emitters, such as Lu, that are being used to label antisomatostatin receptor peptides for neuroendocrine cancers and also prostate-specific membrane antigen targeting small molecules for prostate cancer. Alpha-particle-emitting radionuclides have also been studied for radiopharmaceutical therapy; these include At for glioblastoma, Ac for leukemias and prostate cancer, Pb for breast cancer, and Ra for prostate cancer. The alpha emitters have tended to show particular promise, and there is substantial interest in further developing these agents for therapy of cancers that are particularly difficult to treat.

Targeted and Nontargeted α-Particle Therapies

α-Particle irradiation of cancerous tissue is increasingly recognized as a potent therapeutic option. We briefly review the physics, radiobiology, and dosimetry of α-particle emitters, as well as the distinguishing features that make them unique for radiopharmaceutical therapy. We also review the emerging clinical role of α-particle therapy in managing cancer and recent studies on in vitro and preclinical α-particle therapy delivered by antibodies, other small molecules, and nanometer-sized particles. In addition to their unique radiopharmaceutical characteristics, the increased availability and improved radiochemistry of α-particle radionuclides have contributed to the growing recent interest in α-particle radiotherapy. Targeted therapy strategies have presented novel possibilities for the use of α-particles in the treatment of cancer. Clinical experience has already demonstrated the safe and effective use of α-particle emitters as potent tumor-selective drugs for the treatment of leukemia and metastatic disease.

Effective treatment of ductal carcinoma in situ with a HER-2- targeted alpha-particle emitting radionuclide in a preclinical model of human breast cancer

The standard treatment for ductal carcinoma in situ (DCIS) of the breast is surgical resection, followed by radiation. Here, we tested localized therapy of DCIS in mice using the immunoconjugate ²²⁵Ac linked-trastuzumab delivered through the intraductal (i.duc) route. Trastuzumab targets HER-2/neu, while the alpha-emitter ²²⁵Ac (half-life, 10 days) delivers highly cytotoxic, focused doses of radiation to tumors. Systemic ²²⁵Ac, however, elicits hematologic toxicity and at high doses free ²¹³Bi, generated by its decay, causes renal toxicity. I.duc delivery of the radioimmunoconjugate could bypass its systemic toxicity. Bioluminescent imaging showed that the therapeutic efficacy of intraductal ²²⁵Ac-trastuzumab (10-40 nCi per mammary gland; 30-120 nCi per mouse) in a DCIS model of human SUM225 cancer cells in NSG mice was significantly higher (p<0.0003) than intravenous (120 nCi per mouse) administration, with no kidney toxicity or loss of body weight. Our findings suggest that i.duc radioimmunotherapy using ²²⁵Ac-trastuzumab deserves greater attention for future clinical development as a treatment modality for early breast cancer.

Pharmacokinetics, microscale distribution, and dosimetry of alpha-emitter-labeled anti-PD-L1 antibodies in an immune competent transgenic breast cancer model

BACKGROUND

Studies combining immune checkpoint inhibitors with external beam radiation have shown a therapeutic advantage over each modality alone. The purpose of these works is to evaluate the potential of targeted delivery of high LET radiation to the tumor microenvironment via an immune checkpoint inhibitor.

METHODS

The impact of protein concentration on the distribution of 111In-DTPA-anti-PD-L1-BC, an 111In-antibody conjugate targeted to PD-L1, was evaluated in an immunocompetent mouse model of breast cancer. 225Ac-DOTA-anti-PD-L1-BC was evaluated by both macroscale (ex vivo biodistribution) and microscale (alpha-camera images at a protein concentration determined by the 111In data.

RESULTS

The evaluation of 111In-DTPA-anti-PD-L1-BC at 1, 3, and 10 mg/kg highlighted the impact of protein concentration on the distribution of the labeled antibody, particularly in the blood, spleen, thymus, and tumor. Alpha-camera images for the microscale distribution of 225Ac-DOTA-anti-PD-L1-BC showed a uniform distribution in the liver while highly non-uniform distributions were obtained in the thymus, spleen, kidney, and tumor. At an antibody dose of 3 mg/kg, the liver was dose-limiting with an absorbed dose of 738 mGy/kBq; based upon blood activity concentration measurements, the marrow absorbed dose was 29 mGy/kBq.

CONCLUSIONS

These studies demonstrate that 225Ac-DOTA-anti-PD-L1-BC is capable of delivering high LET radiation to PD-L1 tumors. The use of a surrogate SPECT agent, 111In-DTPA-anti-PD-L1-BC, is beneficial in optimizing the dose delivered to the tumor sites. Furthermore, an accounting of the microscale distribution of the antibody in preclinical studies was essential to the proper interpretation of organ absorbed doses and their likely relation to biologic effect.

Generation of nephron progenitor cells and kidney organoids from human pluripotent stem cells

A variety of protocols have been developed that demonstrate the capability to differentiate human pluripotent stem cells (hPSCs) into kidney structures. Our goal was to develop a high-efficiency protocol to generate nephron progenitor cells (NPCs) and kidney organoids to facilitate applications for tissue engineering, disease modeling and chemical screening. Here, we describe a detailed protocol resulting in high-efficiency production (80-90%) of NPCs from hPSCs within 9 d of differentiation. Kidney organoids were generated from NPCs within 12 d with high reproducibility using 96-well plates suitable for chemical screening. The protocol requires skills for culturing hPSCs and careful attention to morphological changes indicative of differentiation. This kidney organoid system provides a platform for studies of human kidney development, modeling of kidney diseases, nephrotoxicity and kidney regeneration. The system provides a model for in vitro study of kidney intracellular and intercompartmental interactions using differentiated human cells in an appropriate nephron and stromal context.

Improved safety and efficacy of 213Bi-DOTATATE-targeted alpha therapy of somatostatin receptor-expressing neuroendocrine tumors in mice pre-treated with L-lysine

Background

Targeted alpha therapy (TAT) offers advantages over current β-emitting conjugates for peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors. PRRT with ¹⁷⁷Lu-DOTATATE or ⁹⁰Y-DOTATOC has shown dose-limiting nephrotoxicity due to radiopeptide retention in the proximal tubules. Pharmacological protection can reduce renal uptake of radiopeptides, e.g., positively charged amino acids, to saturate in the proximal tubules, thereby enabling higher radioactivity to be safely administered. The aim of this preclinical study was to evaluate the therapeutic effect of ²¹³Bi-DOTATATE with and without renal protection using L-lysine in mice. Tumor uptake and kinetics as a function of injected mass of peptide (range 0.03–3 nmol) were investigated using ¹¹¹In-DOTATATE. These results allowed estimation of the mean radiation absorbed tumor dose for ²¹³Bi-DOTATATE. Pharmacokinetics and dosimetry of ²¹³Bi-DOTATATE was determined in mice, in combination with renal protection. A dose escalation study with ²¹³Bi-DOTATATE was performed to determine the maximum tolerated dose (MTD) with and without pre-administration of l-lysine as for renal protection. Neutrophil gelatinase-associated lipocalin (NGAL) served as renal biomarker to determine kidney injury.

Results

The maximum mean radiation absorbed tumor dose occurred at 0.03 nmol and the minimum at 3 nmol. Similar mean radiation absorbed tumor doses were determined for 0.1 and 0.3 nmol with a mean radiation absorbed dose of approximately 0.5 Gy/MBq ²¹³Bi-DOTATATE. The optimal mass of injected peptide was found to be 0.3 nmol. Tumor uptake was similar for ¹¹¹In-DOTATATE and ²¹³Bi-DOTATATE at 0.3 nmol peptide. Lysine reduced the renal uptake of ²¹³Bi-DOTATATE by 50% with no effect on the tumor uptake. The MTD was <13.0 ± 1.6 MBq in absence of l-lysine and 21.7 ± 1.9 MBq with l-lysine renal protection, both imparting an LD₅₀ mean renal radiation absorbed dose of 20 Gy. A correlation was found between the amount of injected radioactivity and NGAL levels.

Conclusions

The therapeutic potential of ²¹³Bi-DOTATATE was illustrated by significantly decreased tumor burden and improved overall survival. Renal protection with l-lysine immediately prior to TAT with ²¹³Bi-DOTATATE prolonged survival providing substantial evidence for pharmacological nephron blockade to mitigate nephrotoxicity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0240-5) contains supplementary material, which is available to authorized users.

(2S)-2-(3-(1-Carboxy-5-(4-211At-Astatobenzamido)Pentyl)Ureido)-Pentanedioic Acid for PSMA-Targeted α-Particle Radiopharmaceutical Therapy

Alpha-particle emitters have a high linear energy transfer and short range, offering the potential for treating micrometastases while sparing normal tissues. We developed a urea-based, ²¹¹At-labeled small molecule targeting prostate-specific membrane antigen (PSMA) for the treatment of micrometastases due to prostate cancer (PC).

METHODS

PSMA-targeted (2S)-2-(3-(1-carboxy-5-(4-²¹¹At-astatobenzamido)pentyl)ureido)-pentanedioic acid (²¹¹At- 6: ) was synthesized. Cellular uptake and clonogenic survival were tested in PSMA-positive (PSMA+) PC3 PIP and PSMA-negative (PSMA-) PC3 flu human PC cells after ²¹¹At- 6: treatment. The antitumor efficacy of ²¹¹At- 6: was evaluated in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts at a 740-kBq dose and in mice bearing PSMA+, luciferase-expressing PC3-ML micrometastases. Biodistribution was determined in mice bearing PSMA+ PC3 PIP and PSMA- PC3 flu flank xenografts. Suborgan distribution was evaluated using α-camera images, and microscale dosimetry was modeled. Long-term toxicity was assessed in mice for 12 mo.

RESULTS

²¹¹At- 6: treatment resulted in PSMA-specific cellular uptake and decreased clonogenic survival in PSMA+ PC3 PIP cells and caused significant tumor growth delay in PSMA+ PC3 PIP flank tumors. Significantly improved survival was achieved in the newly developed PSMA+ micrometastatic PC model. Biodistribution showed uptake of ²¹¹At- 6: in PSMA+ PC3 PIP tumors and in kidneys. Microscale kidney dosimetry based on α-camera images and a nephron model revealed hot spots in the proximal renal tubules. Long-term toxicity studies confirmed that the dose-limiting toxicity was late radiation nephropathy.

CONCLUSION

PSMA-targeted ²¹¹At- 6: α-particle radiotherapy yielded significantly improved survival in mice bearing PC micrometastases after systemic administration. ²¹¹At- 6: also showed uptake in renal proximal tubules resulting in late nephrotoxicity, highlighting the importance of long-term toxicity studies and microscale dosimetry.

Contribution of Auger/conversion electrons to renal side effects after radionuclide therapy: preclinical comparison of (161)Tb-folate and (177)Lu-folate

Background

The radiolanthanide ¹⁶¹Tb has, in recent years, attracted increasing interest due to its favorable characteristics for medical application. ¹⁶¹Tb exhibits similar properties to the widely-used therapeutic radionuclide ¹⁷⁷Lu. In contrast to ¹⁷⁷Lu, ¹⁶¹Tb yields a significant number of short-ranging Auger/conversion electrons (≤50 keV) during its decay process. ¹⁶¹Tb has been shown to be more effective for tumor therapy than ¹⁷⁷Lu if applied using the same activity. The purpose of this study was to investigate long-term damage to the kidneys after application of ¹⁶¹Tb-folate and compare it to the renal effects caused by ¹⁷⁷Lu-folate.

Methods

Renal side effects were investigated in nude mice after the application of different activities of ¹⁶¹Tb-folate (10, 20, and 30 MBq per mouse) over a period of 8 months. Renal function was monitored by the determination of ^99mTc-DMSA uptake in the kidneys and by measuring blood urea nitrogen and creatinine levels in the plasma. Histopathological analysis was performed by scoring of the tissue damage observed in HE-stained kidney sections from euthanized mice.

Results

Due to the co-emitted Auger/conversion electrons, the mean absorbed renal dose of ¹⁶¹Tb-folate (3.0 Gy/MBq) was about 24 % higher than that of ¹⁷⁷Lu-folate (2.3 Gy/MBq). After application of ¹⁶¹Tb-folate, kidney function was reduced in a dose- and time-dependent manner, as indicated by the decreased renal uptake of ^99mTc-DMSA and the increased levels of blood urea nitrogen and creatinine. Similar results were obtained when ¹⁷⁷Lu-folate was applied at the same activity. Histopathological investigations confirmed comparable renal cortical damage after application of the same activities of ¹⁶¹Tb-folate and ¹⁷⁷Lu-folate. This was characterized by collapsed tubules and enlarged glomeruli with fibrin deposition in moderately injured kidneys and glomerulosclerosis in severely damaged kidneys.

Conclusions

Tb-folate induced dose-dependent radionephropathy over time, but did not result in more severe damage than ¹⁷⁷Lu-folate when applied at the same activity. These data are an indication that Auger/conversion electrons do not exacerbate overall renal damage after application with ¹⁶¹Tb-folate as compared to ¹⁷⁷Lu-folate, even though they result in an increased dose deposition in the renal tissue. Global toxicity affecting other tissues than kidneys remains to be investigated after ¹⁶¹Tb-based therapy, however.

Influence of tumour size on the efficacy of targeted alpha therapy with (213)Bi-[DOTA(0),Tyr(3)]-octreotate

Background

Targeted alpha therapy has been postulated to have great potential for the treatment of small clusters of tumour cells as well as small metastases. ²¹³Bismuth, an α-emitter with a half-life of 46 min, has shown to be effective in preclinical as well as in clinical applications. In this study, we evaluated whether ²¹³Bi-[DOTA⁰, Tyr³]-octreotate (²¹³Bi-DOTATATE), a ²¹³Bi-labelled somatostatin analogue with high affinity for somatostatin receptor subtype 2 (SSTR₂), is suitable for the treatment of larger neuroendocrine tumours overexpressing SSTR₂ in comparison to its effectiveness for smaller tumours. We performed a preclinical targeted radionuclide therapy study with ²¹³Bi-DOTATATE in animals bearing tumours of different sizes (50 and 200 mm³) using two tumour models: H69 (human small cell lung carcinoma) and CA20948 (rat pancreatic tumour).

Methods

Pharmacokinetics was determined for calculation of dosimetry in organs and tumours. H69- or CA20948-xenografted mice with tumour volumes of approximately 120 mm³ were euthanized at 10, 30, 60 and 120 min post injection of a single dose of ²¹³Bi-DOTATATE (1.5–4.8 MBq). To investigate the therapeutic efficacy of ²¹³Bi-DOTATATE, xenografted H69 and CA20948 tumour-bearing mice with tumour sizes of 50 and 200 mm³ were administered daily with a therapeutic dose of ²¹³Bi-DOTATATE (0.3 nmol, 2–4 MBq) for three consecutive days. The animals were followed for 90 days after treatment. At day 90, mice were injected with 25 MBq ^99mTc-DMSA and imaged by SPECT/CT to investigate possible renal dysfunction due to ²¹³Bi-DOTATATE treatment.

Results

Higher tumour uptakes were found in CA20948 tumour-bearing animals compared to those in H69 tumour-bearing mice with the highest tumour uptake of 19.6 ± 6.6 %IA/g in CA20948 tumour-bearing animals, while for H69 tumour-bearing mice, the highest tumour uptake was found to be 9.8 ± 2.4 %IA/g. Nevertheless, as the anti-tumour effect was more pronounced in H69 tumour-bearing mice, the survival rate was higher. Furthermore, in the small tumour groups, no regrowth of tumour was found in two H69 tumour-bearing mice and in one of the CA20948 tumour-bearing mice. No renal dysfunction was observed in ²¹³Bi-DOTATATE-treated mice after the doses were applied.

Conclusions

²¹³Bi-DOTATATE demonstrated a great therapeutic effect in both small and larger tumour lesions. Higher probability for stable disease was found in animals with small tumours. ²¹³Bi-DOTATATE was effective in different neuroendocrine (H69 and CA20948) tumour models with overexpression of SSTR₂ in mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0162-2) contains supplementary material, which is available to authorized users.

Auger Radiopharmaceutical Therapy Targeting Prostate-Specific Membrane Antigen

Auger electron emitters such as (125)I have a high linear energy transfer and short range of emission (<10 μm), making them suitable for treating micrometastases while sparing normal tissues. We used a highly specific small molecule targeting the prostate-specific membrane antigen (PSMA) to deliver (125)I to prostate cancer cells.

METHODS

The PSMA-targeting Auger emitter 2-[3-[1-carboxy-5-(4-(125)I-iodo-benzoylamino)-pentyl]-ureido]-pentanedioic acid ((125)I-DCIBzL) was synthesized. DNA damage (via phosphorylated H2A histone family member X staining) and clonogenic survival were tested in PSMA-positive (PSMA+) PC3 PIP and PSMA-negative (PSMA-) PC3 flu human prostate cancer cells after treatment with (125)I-DCIBzL. Subcellular drug distribution was assessed with confocal microscopy using a related fluorescent PSMA-targeting compound YC-36. In vivo antitumor efficacy was tested in nude mice bearing PSMA+ PC3 PIP or PSMA- PC3 flu flank xenografts. Animals were administered (intravenously) 111 MBq (3 mCi) of (125)I-DCIBzL, 111 MBq (3 mCi) of (125)I-NaI, an equivalent amount of nonradiolabeled DCIBzL, or saline.

RESULTS

After treatment with (125)I-DCIBzL, PSMA+ PC3 PIP cells exhibited increased DNA damage and decreased clonogenic survival when compared with PSMA- PC3 flu cells. Confocal microscopy of YC-36 showed drug distribution in the perinuclear area and plasma membrane. Animals bearing PSMA+ PC3 PIP tumors had significant tumor growth delay after treatment with (125)I-DCIBzL, with only 1 mouse reaching 5 times the initial tumor volume by 60 d after treatment, compared with a median time to 5 times volume of less than 15 d for PSMA- PC3 flu tumors and all other treatment groups (P = 0.002 by log-rank test).

CONCLUSION

PSMA-targeted radiopharmaceutical therapy with the Auger emitter (125)I-DCIBzL yielded highly specific antitumor efficacy in vivo, suggesting promise for treatment of prostate cancer micrometastases.

Dosimetry for radiopharmaceutical therapy

Radiopharmaceutical therapy (RPT) involves the use of radionuclides that are either conjugated to tumor-targeting agents (e.g., nanoscale constructs, antibodies, peptides, and small molecules) or concentrated in tissue through natural physiological mechanisms that occur predominantly in neoplastic or otherwise targeted cells (e.g., Graves disease). The ability to collect pharmacokinetic data by imaging and use this to perform dosimetry calculations for treatment planning distinguishes RPT from other systemic treatment modalities such as chemotherapy, wherein imaging is not generally used. Treatment planning has not been widely adopted, in part, because early attempts to relate dosimetry to outcome were not successful. This was partially because a dosimetry methodology appropriate to risk evaluation rather than efficacy and toxicity was being applied to RPT. The weakest links in both diagnostic and therapeutic dosimetry are the accuracy of the input and the reliability of the radiobiological models used to convert dosimetric data to the relevant biologic end points. Dosimetry for RPT places a greater demand on both of these weak links. To date, most dosimetric studies have been retrospective, with a focus on tumor dose-response correlations rather than prospective treatment planning. In this regard, transarterial radioembolization also known as intra-arterial radiation therapy, which uses radiolabeled ((90)Y) microspheres of glass or resin to treat lesions in the liver holds much promise for more widespread dosimetric treatment planning. The recent interest in RPT with alpha-particle emitters has highlighted the need to adopt a dosimetry methodology that specifically accounts for the unique aspects of alpha particles. The short range of alpha-particle emitters means that in cases in which the distribution of activity is localized to specific functional components or cell types of an organ, the absorbed dose will be equally localized and dosimetric calculations on the scale of organs or even voxels (~5mm) are no longer sufficient. This limitation may be overcome by using preclinical models to implement macromodeling to micromodeling. In contrast to chemotherapy, RPT offers the possibility of evaluating radiopharmaceutical distributions, calculating tumor and normal tissue absorbed doses, and devising a treatment plan that is optimal for a specific patient or specific group of patients.

A small-scale anatomical dosimetry model of the liver

Radionuclide therapy is a growing and promising approach for treating and prolonging the lives of patients with cancer. For therapies where high activities are administered, the liver can become a dose-limiting organ; often with a complex, non-uniform activity distribution and resulting non-uniform absorbed-dose distribution. This paper therefore presents a small-scale dosimetry model for various source-target combinations within the human liver microarchitecture. Using Monte Carlo simulations, Medical Internal Radiation Dose formalism-compatible specific absorbed fractions were calculated for monoenergetic electrons; photons; alpha particles; and (125)I, (90)Y, (211)At, (99m)Tc, (111)In, (177)Lu, (131)I and (18)F. S values and the ratio of local absorbed dose to the whole-organ average absorbed dose was calculated, enabling a transformation of dosimetry calculations from macro- to microstructure level. For heterogeneous activity distributions, for example uptake in Kupffer cells of radionuclides emitting low-energy electrons ((125)I) or high-LET alpha particles ((211)At) the target absorbed dose for the part of the space of Disse, closest to the source, was more than eight- and five-fold the average absorbed dose to the liver, respectively. With the increasing interest in radionuclide therapy of the liver, the presented model is an applicable tool for small-scale liver dosimetry in order to study detailed dose-effect relationships in the liver.

The role of preclinical models in radiopharmaceutical therapy

Radiopharmaceutical therapy (RPT) is a treatment modality that involves the use of radioactively labeled targeting agents to deliver a cytotoxic dose of radiation to tumor while sparing normal tissue. The biologic function of the target and the biologic action of the targeting agent is largely irrelevant as long as the targeting agent delivers cytotoxic radiation to the tumor. Preclinical RPT studies use imaging and ex vivo evaluation of radioactivity concentration in target and normal tissues to obtain biodistribution and pharmacokinetic data that can be used to evaluate radiation absorbed doses. Since the efficacy and toxicity of RPT depend on radiation absorbed dose, this quantity can be used to translate results from preclinical studies to human studies. The absorbed dose can also be used to customize therapy to account for pharmacokinetic and other differences among patients so as to deliver a prespecified absorbed dose to the tumor or to dose-limiting tissue. The combination of RPT with other agents can be investigated and optimized by identifying the effect of other agents on tumor or normal tissue radiosensitivity and also on how other agents change the absorbed dose to these tissues. RPT is a distinct therapeutic modality whose mechanism of action is well understood. Measurements can be made in preclinical models to help guide clinical implementation of RPT and optimize combination therapy using RPT.

Redefining relative biological effectiveness in the context of the EQDX formalism: implications for alpha-particle emitter therapy

Alpha-particle radiopharmaceutical therapy (αRPT) is currently enjoying increasing attention as a viable alternative to chemotherapy for targeting of disseminated micrometastatic disease. In theory, αRPT can be personalized through pre-therapeutic imaging and dosimetry. However, in practice, given the particularities of α-particle emissions, a dosimetric methodology that accurately predicts the thresholds for organ toxicity has not been reported. This is in part due to the fact that the biological effects caused by α-particle radiation differ markedly from the effects caused by traditional external beam (photon or electron) radiation or β-particle emitting radiopharmaceuticals. The concept of relative biological effectiveness (RBE) is used to quantify the ratio of absorbed doses required to achieve a given biological response with alpha particles versus a reference radiation (typically a beta emitter or external beam radiation). However, as conventionally defined, the RBE varies as a function of absorbed dose and therefore a single RBE value is limited in its utility because it cannot be used to predict response over a wide range of absorbed doses. Therefore, efforts are underway to standardize bioeffect modeling for different fractionation schemes and dose rates for both nuclear medicine and external beam radiotherapy. Given the preponderant use of external beams of radiation compared to nuclear medicine in cancer therapy, the more clinically relevant quantity, the 2 Gy equieffective dose, EQD2(α/β), has recently been proposed by the ICRU. In concert with EQD2(α/β), we introduce a new, redefined RBE quantity, named RBE2(α/β), as the ratio of the two linear coefficients that characterize the α particle absorbed dose-response curve and the low-LET megavoltage photon 2 Gy fraction equieffective dose-response curve. The theoretical framework for the proposed new formalism is presented along with its application to experimental data obtained from irradiation of a breast cancer cell line. Radiobiological parameters are obtained using the linear quadratic model to fit cell survival data for MDA-MB-231 human breast cancer cells that were irradiated with either α particles or a single fraction of low-LET (137)Cs γ rays. From these, the linear coefficient for both the biologically effective dose (BED) and the EQD2(α/β) response lines were derived for fractionated irradiation. The standard RBE calculation, using the traditional single fraction reference radiation, gave RBE values that ranged from 2.4 for a surviving fraction of 0.82-6.0 for a surviving fraction of 0.02, while the dose-independent RBE2(4.6) value was 4.5 for all surviving fraction values. Furthermore, bioeffect modeling with RBE2(α/β) and EQD2(α/β) demonstrated the capacity to predict the surviving fraction of cells irradiated with acute and fractionated low-LET radiation, α particles and chronic exponentially decreasing dose rates of low-LET radiation. RBE2(α/β) is independent of absorbed dose for α-particle emitters and it provides a more logical framework for data reporting and conversion to equieffective dose than the conventional dose-dependent definition of RBE. Moreover, it provides a much needed foundation for the ongoing development of an α-particle dosimetry paradigm and will facilitate the use of tolerance dose data available from external beam radiation therapy, thereby helping to develop αRPT as a single modality as well as for combination therapies.